The present invention is directed to a device for the implementation of a curing process at a semiconductor wafer.
In order to cure crystal defects that can arise, for example, during implantation, it is known to subject semiconductor wafers to a heat treatment process. In curing the implanted layers, the doping profiles that have been set must be largely preserved and the layer structures should not bleed. Further, the formation of the eliminations is suppressed in the case of higher dopings.
Methods that meet these requirements are known (see, for example, D. O. Sedgwick, Short Time Annealing, ECS-Fall Meeting, October 1982). Rapid thermal processing (RTP) is a method wherein only the semiconductor wafer to be cured and, potentially, a reference wafer are heated to a high temperature. The heating proceeds extremely quickly with temperature gradients of, for example, 10.sup.3 .degree. C./sec. The semiconductor wafers are exposed to a high temperature for a brief period of time and are subsequently cooled. For example, the wafers may be exposed to a temperature of 900.degree. C. for 5 to 20 seconds.
The semiconductor wafer can be heated by, for example, the light energy of a strong light source such as, for example, a laser or a lamp. The semiconductor wafer can also be heated by an electrical resistance heating element having a low heat capacity. During the process, the semiconductor wafer is located in a reaction chamber. For example, the semiconductor can be located in a reaction chamber constructed from silica glass. The curing process occurs in an inert atmosphere or in a vacuum. In this process, the diffusion-caused redistribution of imperfections is suppressed as a result of the short annealing time. It is thus assured that the aforementioned requirements are met.
In the RTP method there is a large temperature gradient during the curing process between the semiconductor wafer, that is heated to a curing temperature and the chamber wall that is at ambient temperature. Therefore, when semiconductor material evaporates from the semiconductor wafer, it precipitates onto the chamber wall. When the semiconductor wafer is heated by light energy, the precipitation causes temperature inhomogeneities because the light is partially absorbed by the precipitate. This results in the acquisition and control of the temperature by pyrometers to be faulty. Furthermore, because of the temperature inhomogeneities, the heating process is retarded.
A further problem in curing semiconductor wafers occurs due to the construction of the semiconductors. Semiconductors constructed from two or more elements (i.e., a compound), especially III-V compounds, present problems in curing. Since the elements of the compound have different vapor pressures, the more volatile element escapes first. This leads to an undesirable surface erosion of the semiconductor wafers.
This disadvantage can be alleviated when the semiconductor wafer to be cured is embedded, for example, between two semiconductor wafers or silica plates that are also heated to the curing temperature (see, for example, H. Kanber et al, Appl. Phys. Lett. 47 (2), July 1985, pages 120 ff.). This process reduces the surface erosion. However, this procedure increases the heat capacity of the system and thus the heat flow into the semiconductor wafer becomes more difficult.
Surface erosion can also be avoided by protecting the semiconductor wafer with a cover layer of, for example, Si.sub.3 N.sub.4, during curing (see, for example, H. Kanber et al, Appl. Phys. Lett. 47 (2), July 1985, pages 120 ff.; M. Kuzuhara et al, Mat. Res. Soc. Symp. Proc. Vol. 23 (1984), pages 651 ff.). This procedure, however, has the disadvantage that mechanical tensions and the redistribution of imperfections occur in the semiconductor wafer.
Surface erosion can also be avoided, without the use of a cover layer, in what is referred to as the capless annealing method (see, for example, J. D. Grange et al, Solid State Electronics, Vol. 26, No. 4, 1983, pages 313 ff.). Pursuant to this method, the curing process occurs in an atmosphere that contains an over-pressure of the more volatile element. The over-pressure prevents an evaportion of the more volatile component of the semiconductor. The over-pressure arises by splitting a gaseous hydrogen compound of the more volatile element that is contained in the protective atmosphere. The splitting of the gaseous hydrogen compound is carried out at the curing temperature. This method has the disadvantage that there is a diffusion of imperfections and, thus, a partial destruction of the doping profiles that arises during the heating times required by the method (the curing time: is approximately 10-60 minutes; and the heating time: is approximately 15 minutes).
A combination of the RTP method with the capless annealing method, wherein the reaction chamber in the RTP method is filled with the afore-mentioned hydrogen compound, is not presently viable. This is due to the fact that the heating and curing time in the RTP method ar too short to split the hydrogen compound and thus create an over-pressure of the more volatile component.
There is therefore a need for an improved method and device for curing semiconductor wafers.